I've never tried selling cards at craft fairs, but I have a pretty large family and had an open house displaying my cards in September. I received so many orders for Christmas cards and was thrilled that they actually realized the value of the cards (I was really afraid because most of them buy those boxes of 100 cards for $5 or whatever it is).
I took the time to explain what each card entailed (I had 3 different price ranges for my cards) and allowed them to customize certain aspects. I also had special offers - buy 10 cards (same value) and get 2 free. Buy 20 cards, get 4 free, plus a stamped memo pad. I also mentioned this at work and sold Christmas cards there too !!!
I just think that you have more of an opportunity when you have the chance to explain what it takes to make a card and get to work with the people on making it the card of their dreams !!!